A condenser microphone includes a condenser microphone unit having a diaphragm and a fixed electrode facing the diaphragm. The condenser microphone unit is an acoustoelectric transducer generating electrical signals converted from a variation in the electrostatic capacity of a capacitor defined by the diaphragm and the fixed electrode in response to vibrations of the diaphragm. That is, vibrations of the diaphragm due to sound waves vary the electrostatic capacity to convert the variation in the electrostatic capacity into electrical signals to be output. The condenser microphone unit therefore has a signal source impedance equivalent to the electrostatic capacity of the capacitor. As a result, the condenser microphone needs an impedance converter having extremely high input impedance at the subsequent stage of the condenser microphone unit. The impedance converter is usually composed of a field effect transistor (FET). For example, the condenser microphone unit has a fixed electrode connected to the gate of the FET and has a grounded diaphragm. Known techniques for acquiring signal output from a condenser microphone including an impedance converter having an FET include: grounding the source of the FET and acquiring signal output from the drain (refer to Japanese Unexamined Patent Application Publication No. H8-33090); and grounding the drain of the FET and acquiring signal output from the source.
The technique acquiring signal output from the drain of the FET is called a two wire system or a plug-in power system. The two wire systems are used for many simple microphones. The technique acquiring signal output from the source of the FET is called a three wire system or a source follower. The three wire system can have small distortion and a high dynamic range of output signals in comparison with the two wire system. As a result, the three wire systems are usually used for microphones for sound collection in studios.
These two techniques will now be described with reference to the accompanying drawings illustrating example circuitry. FIG. 8 is a circuit diagram illustrating an example condenser microphone of the two wire system. FIG. 9 is a circuit diagram illustrating an example three wire condenser microphone.
With reference to FIG. 8, a condenser microphone 100 of the two wire system includes a power source circuit 105 supplying operational power to a condenser microphone unit 101 and an impedance converter 102 through a single-core shielded wire 106. A power source Vcc, included in the power source circuit 105, is connected to the core wire of the single-core shielded wire 106 through a load resistor RL. A grounding line for the condenser microphone unit 101 and the impedance converter 102 is connected to a grounding line for the power source circuit 105 by the shield of the single-core shielded wire 106. In other words, the core wire of the single-core shielded wire 106 serves as both a power source line and a signal line connected to the drain of the FET in the impedance converter 102.
With reference to FIG. 9, a three wire condenser microphone 100a includes a power source circuit 105a supplying operational power to a condenser microphone unit 101 and an impedance converter 102 through a double-core shielded wire 106a. The power source Vcc included in the power source circuit 105a is connected to the drain of the FET in the impedance converter through one core wire of the double-core shielded wire 106a. This core wire serves as a power source line. A grounding line included in the power source circuit 105a is connected to the other core wire of the double-core shielded wire 106a through a load resistor RL. The other core wire is connected to the source of the FET in the impedance converter 102 and serves as a signal line. A grounding line for the condenser microphone unit 101 and the grounding line for the power source circuit 105a are connected by the shield of the double-core shielded wire 106a. 
As illustrated in FIG. 8, a condenser microphone of a two wire system including a single-core shielded wire can be composed of a simple circuit. Unfortunately, such a condenser microphone of a two wire system acquiring signal output from the drain of the FET in the impedance converter 102 has high output impedance and often leads to distortion of signals. In comparison with the condenser microphone of the two wire system, the three wire condenser microphone illustrated in FIG. 9 has small distortion and a high dynamic range of signals in exchange for complicated circuitry. It is preferred that the two wire condenser microphone would have advantages of the three wire system exemplified in FIG. 9.
In other words, the two wire condenser microphone composed of simple circuitry should preferably have small distortion and a high dynamic range of output signals.